A voltage reference is a circuit used to provide a reference voltage signal to a circuit. The circuit uses the reference voltage signal as a means of comparison during operation. For example, in voltage regulator applications a feedback signal is compared against the reference voltage in order to create a regulated output voltage corresponding to a scaled value of the voltage reference.
In some approaches, the voltage reference is formed using bipolar junction transistors (BJTs) to form bandgap references to provide the reference voltage signal. In PNP BJTs, the substrate acts as a collector for the BJT thereby rendering the BJT sensitive to majority carrier noise in the substrate. In NPN BJTs, the collector is formed as an n-well in a p-type substrate and is susceptible to picking up minority carrier noise from the substrate. Neither NPN BJTs nor PNP BJTs allow full isolation from substrate noise.
In some approaches, complementary metal oxide semiconductor (CMOS) devices are used to form the voltage reference. In some instances, the CMOS devices are fabricated in a triple well flow such that every CMOS device is reverse-junction-isolated from the main substrate. In some approaches, a CMOS device includes a polysilicon gate feature which is doped using an opposite dopant type from a dopant in the substrate for the CMOS device.